Although the physical mechanism behind drag reduction is simple, SHSs seldom achieve the high drag-reduction performance predicted by theory [1]}, [2]}.
A number of practical difficulties can explain the lower performance measured experimentally, such as displacement of the liquid–gas interfaces [3]}, [4]}, contact-angle effects [5]}, [6]}, viscous drag from the gas phase [7]}, [8]}, as well as surfactant-induced Marangoni stresses [9]}, [10]}.
